Size limits of magnetic-domain engineering in continuous in-plane exchange-bias prototype films

Background: The application of superparamagnetic particles as biomolecular transporters in microfluidic systems for lab-on-a-chip applications crucially depends on the ability to control their motion. One approach for magnetic-particle motion control is the superposition of static magnetic stray field landscapes (MFLs) with dynamically varying external fields. These MFLs may emerge from magnetic domains engineered both in shape and in their local anisotropies. Motion control of smaller beads does necessarily need smaller magnetic patterns, i.e., MFLs varying on smaller lateral scales. The achievable size limit of engineered magnetic domains depends on the magnetic patterning method and on the magnetic anisotropies of the material system. Smallest patterns are expected to be in the range of the domain wall width of the particular material system. To explore these limits a patterning technology is needed with a spatial resolution significantly smaller than the domain wall width. Results: We demonstrate the application of a helium ion microscope with a beam diameter of 8 nm as a mask-less method for local domain patterning of magnetic thin-film systems. For a prototypical in-plane exchange-bias system the domain wall width has been investigated as a function of the angle between unidirectional anisotropy and domain wall. By shrinking the domain size of periodic domain stripes, we analyzed the influence of domain wall overlap on the domain stability. Finally, by changing the geometry of artificial two-dimensional domains, the influence of domain wall overlap and domain wall geometry on the ultimate domain size in the chosen system was analyzed. Conclusion: The application of a helium ion microscope for magnetic patterning has been shown. It allowed for exploring the fundamental limits of domain engineering in an in-plane exchange-bias thin film as a prototypical system. For two-dimensional domains the limit depends on the domain geometry. The relative orientation between domain wall and anisotropy axes is a crucial parameter and therefore influences the achievable minimum domain size dramatically.

Domain Walls in the Early Universe and Generation of Matter and Antimatter Domains

We present a model where it is possible to generate cosmologically large domains of matter and antimatter separated by cosmologically large distances. Domain walls existed only in the early universe and later they disappeared. So the problem of domain walls in this model does not exist. These features are achieved through a postulated form of interaction between inflaton and a new scalar field. This scenario inspired a study of the related problem - evolution of the domain wall width in expanding universe. According to classical results there is a region of parameter space where the solutions with constant physical width exist. Numerical study of the problem demonstrates that initial configurations tend to these solutions with time. However, we have found that the wall width can grow exponentially outside of that parameter region.

Micromagnetic Simulation on Ground State Domain Structures of Barium Hexaferrite (BaFe12O19)

We have systematically investigated the domain structures of Barium Hexaferrite (BaFe12O19) by means of a micromagnetic simulation under zero external magnetic field. A hexagonal-shaped and cylindrical-shaped models are used in this simulation with respect to the diameter variation from 50 nm to 600 nm. A transition domain structure is found from a single-domain (SD) to multi-domain (MD) at a certain diameter. The hexagonal-shaped occurs in diameter 430 nm and cylindrical-shaped in diameter 410 nm. Interestingly, the domain wall of MD structure exhibits a Bloch-wall type. The domain wall width is determined by full width half maximum (FWHM) method from the transverse magnetization component data. The domain wall width from simulation showed close to Kittel’s formula

CURVATURE-INDUCED MAGNETOCHIRALITY

Curved geometries like nanotubes and flexible membranes generally differ from flat films by internal strain, geodesic pathways for transport phenomena, and a break of the local inversion symmetry. In ferromagnetism, these characteristics can lead to surprising effects, especially when the curvature radius reaches intrinsic length scales, like the domain wall width or the magnon wave length. Simulation studies demonstrate that curved ferromagnetic thin films display magnetochiral properties similar to the Dzyaloshinskii–Moriya interaction (DMI). In close analogy to the emerging field of flexoelectricity, it is suggested that the controlled bending of ferromagnetic membranes provides a new, reversible and universal method to manipulate their magnetic properties.